The present invention relates to a rotary member for transmitting a drive force or a braking force of a sprocket, a brake disc or the like.
In general, a mechanism having a pair of sprockets and a change laid therebetween has been widely used as a drive force transmission means in a motorcycle or a bicycle.
Usually, a sprocket is integrally formed of an annular hub portion arranged in the center thereof, an annular ring portion arranged in a circumferential portion thereof, and a plurality of arm portions extending radially and arranged between the hub portion and the ring portion for connecting the hub portion and the ring portion at a constant angular interval in the circumferential direction.
A carbon steel plate has been widely used as a material for the sprocket. In a manufacturing method, after the carbon steel plate has been machined to have a form of the sprocket by mechanical machining or pressing, a tooth portion is annealed to have a desired hardness. Thereafter, a finishing treatment is effected on the sprocket.
Also, in view of an aesthetic design demand, there is a sprocket 100, as shown in FIG. 15, in which a center PR of a distal end portion on a ring portion 102 side and a center PH of a proximal end portion on a hub 103 side of each arm portion 101 are offset at a constant angle in the circumferential direction.
On the other hand, a brake disc in which an annular braking portion is formed in its outer circumferential portion and the braking portion clamped by pad members to generate the braking force has been extensively used as a braking mechanism in a motorcycle or the like.
A so-called floating disc brake mechanism has been widely practically used, in which a braking portion against which pad members are to be pressed and a disc like hub portion externaly fixedly mounted on an axle or the like are formed of discrete members, respectively; a plurality of pin members are mounted between the hub portion and the braking member; the braking portion is floatingly supported to the hub portion; and the braking portion and hub portions are slit at their edge to thereby prevent a warpage of the disc brake caused by heat generated during the braking operation.
Also, in a brake disc 110 as shown in FIG. 16, a hub portion 111 and a braking portion 112 are integrally formed with each other; the hub portion 111 and the braking portion 112 are coupled with each other through a plurality of arm portions 113; and in view of the aesthetic design demand, a center PR of a distal end portion on a braking portion 112 side and a center PH of a proximal end portion on a hub 111 side of each arm portion are offset in the circumferential direction with constant regularity.
In the sprocket in which the arm portions are radially arranged, after the annealing treatment for the tooth portion, the ring portion tends to shrink inwardly, but the hardness of the ring portion per se is high by the annealing treatment so that the shrinkage force is applied to the arm portions and the hub portion as a compression stress. Then, due to the compression stress, such a phenomenon that portions, corresponding to the arm portions, of the hub portion are deformed inwardly is generated. For this reason, after the annealing treatment of the tooth portion, a machining is effected to the inner circumferential surface of the hub portion. A cutting step is required to keep the inner circumferential surface in an exact circle. Thus, there is a disadvantage that the manufacturing steps for the sprocket are complicated.
Also, in order to solve this problem, it is possible to try to form the ring portion and the hub portion from discrete members like the braking portion and the hub portion of the floating disc brake. However, in this case, the number of the components is increased to raise the manufacturing cost corresponding to the additional components.
The present application made and tested various changes in shape of the arm portions and weight reduction holes on the basis of the concept that the compression stress applied to the hub portion was reduced by elastically deforming the arm portions. As a result, the present applicant has found out that the centers of the distal ends of the arm portions on the ring portion side and the centers of the proximal ends of the arm portions on the hub portion side are offset in one circumferential direction with a constant regularity, respectively, so that the deformation of the inner circumferential surface of the hub portion 102 may be reduced. Also, as shown in FIG. 15, even if the distal ends and the proximal ends of the arm portions 101 are offset in one circumferential direction with a constant regularity, if there are regions where the hub portion 102 and the ring portion 103 are directly connected with each other by longitudinal sectional lines intersecting with the arm portions 101 by hatching, it has been found that the deformation of the inner circumferential surface of the hub portion 102 is remarkable.
Namely, it has been found that not only in view of the design demand but also of the demand on the manufacture, the distal end portions and the proximal end portions of the arm portion are offset in one circumferential direction with a constant regularity, and in addition, the arm portions are formed so that the hub portion and the ring portion are not directly connected to each other by the longitudinal sectional lines intersecting with the arm portions in the radial direction of the sprocket, and in other words, the arm portions and the weight reduction holes are formed so that the lines in the radial direction of the sprocket are always kept to intersect with any weight reduction holes over the entire circumference of the sprocket, whereby it is possible to considerably reduce the deformation of the inner circumferential surface of the hub portion.
Also, even in a disc brake which is integrally made of a single steel plate, after the braking portion is annealed, there occurred a problem that is similar to that of the sprocket. In addition, due to the heat generation during the braking operation, the braking portion is expanded and a tension stress is effected at the arm portion so that the brake disc as a whole is subjected to the warpage. It is also found that the deformation of the inner circumferential surface of the hub portion or the warpage of the brake disc as a whole may be prevented in the same method. In the case where the ring portion is expanded due to the heat generation during the braking operation, this expansion is effectively absorbed by the arm portions.